Method for producing a metal component

ABSTRACT

Method for machining a metal component which has a three-dimensional shape produced by removing and/or shaping material, wherein one or more superior component sections are electrochemically finish-machined by means of a nozzle-like cathode, via which an electrolyte is delivered into the working region, and wherein the cathode or the metal component is moved freely in space by means of a manipulator element.

This application claims priority to German Patent Application No.1020010032701.8 filed Jul. 29, 2010, the contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

The invention relates to a method for machining a metal component whichhas a three-dimensional shape produced by removing and/or shapingmaterial.

Metal components are machined or shaped using various methods in orderto obtain a desired three-dimensional geometry. Material-removingmethods such as, for example, drilling, turning, milling, EDM(electrical-discharge machining) and ECM (electrochemical machining) ormaterial-shaping methods such as, for example, stamping, pressing orforging are known. These methods normally serve for rough contouring,i.e. the three-dimensional shape is substantially fashioned by theseworking processes. Superior component sections, however, still have tobe subjected to finish machining in order for any burrs, projections,edges, corners and the like to be removed, etc. This finish machining iseffected in most cases by milling. Whereas this is oftenstraightforwardly possible in the case of large metal components onaccount of the accessibility of the superior component sections whichare to be finish-machined, problems often arise in particular withsmaller metal components or with metal components in which the superiorcomponent sections are either very narrow or are difficult to reach onaccount of the component geometry, since the tool can be guided into thedesired region only some of the way, if at all. Further problems areoften caused by the material of the metal component. Special alloys,such as, for example, titanium alloys or nickel alloys, are often usedin particular for special applications. Whereas components made oftitanium alloys still have sufficient machinability and can therefore,for example, be satisfactorily milled, components made of nickel alloyshave relatively poor machinability. In conjunction with complexgeometrical relationships, this gives rise to even greater problems inthe course of the finish machining.

SUMMARY OF THE INVENTION

The problem addressed by the invention is therefore to specify a methodwhich is intended for machining a metal component and makes possiblefinish machining of superior component sections even in the case ofcomplex component geometry and difficult machinability of the componentmaterial.

To solve this problem, provision is made in a method of the typedescribed at the beginning for one or more superior component sectionsto be electrochemically finish-machined by means of a nozzle-likecathode, via which an electrolyte is delivered into the working region,wherein the cathode or the metal component is moved freely in space bymeans of a manipulator element.

In the method according to the invention, the finish machining of thesuperior component region or regions, such as in the region of edges,surface transitions, burrs and the like, is effected by electrochemicalmachining, normally called ECM. For this purpose, use is made of anozzle-like cathode, via which the electrolyte is delivered directlyinto the working region. That is to say that the electrolyte is fed tothe cathode and, at the tip of the nozzle-like cathode, flows directlyonto the metal component (workpiece) to be finish-machined. Theapplication of a working voltage between metal component and cathodeproduces a process current which causes material to be removed. Themetal component itself here is anodically polarized. Via the nozzle-likecathode, material can thus be removed from the workpiece in a preciselydefined region, specifically only in the contact region of the impingingelectrolyte stream. According to a first alternative of the invention,the cathode itself is arranged on a manipulator element, preferably amulti-axis robot, preferably one having at least five motion axes, viawhich robot, with associated control device, the cathode can be movedfreely (three-dimensionally) in space. That is to say that the cathodecan be moved in any desired manner and can therefore also traverse anydesired geometries. Since the cathode itself is a very thin, narrowcomponent having a diameter of only a few millimeters, it canconsequently be moved even into extremely narrow, constricted componentregions, and displaced there, via the robot. This in turn makes itpossible even for components having a very complex geometry to bemachined free of machining forces in superior, otherwise scarcelyaccessible regions. According to a second alternative of the invention,the metal component can be moved freely (three-dimensionally) in spaceby means of the manipulator element, again preferably a multi-axisrobot, preferably one having at least five motion axes; that is to saythat, here, the metal component is moved in any desired manner relativeto the thin cathode. As a result of the free mobility, even complexgeometries can be machined by means of this motion variant. Theinvention is therefore based on the idea that there is spatially freerelative mobility between cathode and metal component, and this relativemobility is realized by means of the manipulator element.

The electrochemical working process also enables a wide variety ofdifferent materials to be machined, that is to say that even materialswhich are difficult to machine using conventional working processes,because they are very difficult to cut, can be readily machined by ECM.In conjunction with the freedom of movement of the cathode or of themetal component, that is to say the mobility in any desired manner inspace, the method according to the invention therefore offers thepossibility of being able to finish-machine any desired components orcomplex geometries virtually irrespective of the material used.

As described, the manipulator element provided is a robot which shouldhave preferably five motion axes, but it can equally also have six axes,thereby ultimately providing for even more degrees of freedom ofmovement. This multi-axis configuration enables both translationalmovements in the three space directions and rotational movements aboutthe space directions.

According to a development of the invention, the movement is notrestricted only to the cathode according to the first alternative of theinvention or to the metal component according to the second alternativeof the invention. Rather, it is also possible to move the metalcomponent (with free mobility of the cathode) or the cathode (with freemobility of the metal component) in addition, that is to say to permit,for example, a translational movement, for example along one or morespace axes, or to rotate said metal component or cathode, possibly inaddition, about one or more space axes. That is to say that there areadditional degrees of freedom of movement at the respectively othercentral working element, in addition to the movements which are possiblevia the manipulator element.

As stated, the cathode is a thin tube having a diameter of a fewmillimeters, provided it is round in cross section and delivers a roundelectrolyte stream. Alternatively, an as it were “squeezed” cathode,which is longer than it is wide, or a hollow cathode having any otherdesired cross section can be used. The geometry of the working region isdefined according to the electrolyte stream geometry, for which reasonthe corresponding cathode geometry is selected according to thecomponent section to be machined. In order for workpiece sections whichare difficult to access to be reached more easily, curved or angledcathode embodiments can be used.

The essential process parameters are the distance between cathode andworkpiece, the working voltage or the process current, the dwell time ofthe cathode above the location to be machined or the associated feedrate of the cathode relative to the metal component, and the compositionand the volumetric flow of the electrolyte. By suitable selection ofthese parameters, the material removal can be controlled with regard toremoval depth and removal rate, care always having to be taken whensetting the parameters that as few stray currents as possible occur,which would possibly lead to material also being removed outside theactual working region. For example, it is possible for the workingvoltage which is applied between cathode and metal component, andtypically between 5V and 200V, or the process current flowingtherebetween to be constant or pulsed and/or for the volumetric flow ofthe electrolyte to be between 10 and 1001/h.

The control of the robot, that is to say the movement of the cathode fortraversing the component geometry to be machined, is effected via asuitable control device which has a corresponding control program. Thecontrol is based on a model of the component or of the componentgeometry, said model being stored in a suitable program which serves forthe control. During operation, then, the cathode or the metal component(depending on which is moved by the manipulator element), supported bythe program, is moved along the component section to be machined. Since,as described, the material removal depends ultimately on the set processparameters and in particular on the distance of the cathode from thecomponent surface, an expedient development of the invention providesfor the distance between the cathode and the surface of the metalcomponent to be determined from the ratio of working voltage and processcurrent and/or, as a function thereof, for the process sequence to becontrolled by varying any desired abovementioned process parameter, e.g.the feed rate, it being possible for the feed to be constant orintermittent.

The working voltage and the process current are detected, and thecross-sectional area of the electrolyte stream and thus the working areaare also known. These variables, in first approximation, have a clearlydefined, formal relationship in accordance with Ohm's law, to thedistance between the cathode and the surface of the metal component. Theremoval depth and thus the working progress are therefore clearly linkedto the aforesaid parameters. Any desired parameter, e.g. the dwell timeof the cathode above the metal component surface or the feed rate of thecathode along the component surface, can therefore be set according tothe continuously determined cathode spacing in such a way that thedesired working result is achieved.

In a development of the invention, a gas curtain, preferably an aircurtain, laterally enclosing the electrolyte stream at least partly,preferably completely, is blown out via the cathode. That is to say thatnot only is the electrolyte stream delivered via the cathode but so toois a gas stream, which encloses the electrolyte stream preferablycompletely. The result of this is that the electrolyte stream isdelivered onto the component surface in a concentrated manner and iskept away from the vicinity of the working region by being “blown out”.This avoids the situation where adjacent surfaces are undesirablyaffected and therefore stray currents lead to undesirable removal inadjacent regions. The nozzle itself is therefore of double-walleddesign, with an inner electrolyte passage and an outer air passage,which are connected to corresponding supply lines. In addition to air,some other gas, e.g. inert gas such as nitrogen or helium, can also beused for forming the gas curtain.

As already described, the method according to the invention is suitablein particular for machining components made of special (metallic andintermetallic, high-temperature) materials; it is primarily useful forthe machining of metal components made of steels, titanium alloys and inparticular nickel alloys, which are extremely difficult to cut.

A metal component which is in particular preferably to be machined withthe method according to the invention is a component of a turbomachine,for example a casing component, but in particular a blade component.Blade components which are especially difficult to machine and have avery complex geometry are integral rotors (“blisks”), guide vaneclusters or guide vane rings, as are used, for example, in high-pressurecompressors of a gas turbine. Such integral rotors, guide vane clustersor guide vane rings are subject to stringent requirements, for whichreason they consist either of a titanium alloy or high-temperaturesteel, but preferably of a nickel alloy. In particular the guide vaneclusters and the guide vane rings have a very complex geometry, normallyconsisting of two shrouds, between which the twisted guide airfoilsextend. The distances between the airfoils go from a few millimeters upinto the centimeter range. As a result, the accessibility of the regionsbetween the airfoils is greatly restricted. Nonetheless, in particularthe edges/corners or the transition surfaces in these regions requirethe finish machining according to the invention. If such a guide vanecluster or a guide vane ring is produced from solid material by cuttingor other removal processes, this inevitably results in the finishmachining, in particular in the region between the airfoils, involvingconsiderable outlay. It is precisely in the production of bladecomponents, in particular of the guide vane clusters or guide vanerings, that the method according to the invention can be used in anespecially advantageous manner, in particular if there are very smallairfoil and shroud spacings. This is because, with the method accordingto the invention, the edges and surfaces present there in the regionbetween two airfoils and/or the airfoils themselves can be readilymachined, since the very thin, narrow cathode can be moved even intothese extremely narrow regions, and positioned there with highprecision, via the robot moving it in any desired manner in space.

In addition to the method, the invention also relates to an apparatusfor implementing the method, comprising a manipulator element in theform of a multi-axis, preferably five- or six-axis, robot, on which anECM tool in the form of a nozzle-like cathode or a work holder holdingthe metal component is arranged, an electrolyte feed device for feedingthe electrolyte from an electrolyte reservoir to the cathode, a processenergy source connected to the cathode and the metal component, and acontrol device controlling the operation of the apparatus. The cathode,which of course is interchangeably arranged on a corresponding cathodeholder on the robot, has a round, elongated or any other desired crosssection; the desired cathode shape is dictated by the machining task. Inorder to be able to more easily reach workpiece sections where access isdifficult, curved or angled cathode embodiments are possible.

In a development of the invention, the cathode can be designed fordelivering gas, in particular air, fed to it via a gas feed device, inthe form of an air curtain laterally enclosing the electrolyte stream atleast partly, preferably completely, said electrolyte stream dischargingfrom the cathode. The material removal is thereby concentrated on theworking region, and the effect on adjacent zones can thereby be reduced.The nozzle itself is therefore, for example, of double-walled design,with an inner electrolyte passage and an outer gas passage enclosingsaid electrolyte passage, said passages being connected to correspondingsupply lines.

In an advantageous development, the work holder is additionally movable,preferably along or about a plurality of space axes, when the cathode isarranged on the manipulator element, or the cathode is additionallymovable, preferably likewise along or about a plurality of space axes,when the work holder is arranged on the manipulator element. That is tosay that two movement modalities are provided in the apparatus accordingto the invention, namely, firstly, the robot for moving the cathode orthe work holder together with metal component and, secondly, also thework holder or the cathode, such that an adapted relative motionsequence between cathode (tool) and metal component (workpiece) can beset for the respective application.

Finally, a means for detecting the distance of the cathode from thecomponent surface is provided. This nozzle distance is a measure of theremoval capacity and the removal depth and is clearly linked, in firstapproximation, in accordance with Ohm's law, to the process parametersworking voltage, process current and cross-sectional area of theelectrolyte stream, for which reason the detection of the distance isadvantageous for the continuous monitoring of the working result. Themeans in this respect is expediently the control device, whichdetermines the distance. It is possible for any desired parameter, e.g.the dwell time of the cathode above the workpiece surface or the feedrate of the cathode along the component surface, to be set withreference to the detected distance in such a way that the desiredworking result is achieved. However, the distance can also be determinedby means of one or more distance sensors, the control device againcontrolling the operation, that is to say the relevant processparameters, in accordance with the measuring results from the sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages, features and details of the invention can begathered from the exemplary embodiment described below and withreference to the drawings, in which:

FIG. 1 shows a diagrammatic illustration, as a perspective view, of aguide vane cluster to be machined with the method according to theinvention and the apparatus according to the invention,

FIG. 2 shows a diagrammatic illustration of the apparatus according tothe invention,

FIG. 3 shows a diagrammatic illustration of the cathode/metal componentworking region.

FIG. 4 shows a diagrammatic illustration of the cathode movement, and

FIG. 5 shows a diagrammatic illustration of a cathode with a gas curtainlaterally defining the electrolyte stream.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows, in the form of a perspective view, a metal component 1 inthe form of a guide vane cluster 2 which for the purpose of forming acomplete ring, is assembled with a plurality of such clusters to give aring shape. Such a guide vane cluster consists of two shrouds 3, 4 and amultiplicity of airfoils 5 extending between said shrouds 3, 4. Theshrouds 3, 4 and the airfoils 5 are fashioned from solid material bycutting and/or other removal processes. The airfoils 5 have a complexlytwisted geometry and are very closely spaced apart, i.e. there are onlyvery narrow spaces 6 between the individual airfoils 5. The complexgeometry consequently results in curved edge regions and curved surfacesin the transition between the airfoils 5 and the shrouds 3, 4 or at thesurfaces of the shrouds 3, 4 and at the surfaces of the airfoils 5themselves, which, after the metal component 1 or its sections (shrouds3, 4, airfoils 5) have been pre-machined using appropriate workingprocesses, have to be finish-machined using the method according to theinvention.

An apparatus as shown in FIG. 2 as a diagrammatic illustration is usedfor this purpose. The apparatus comprises a manipulator element 7 in theform of a multi-axis, preferably at least five-axis, robot 8 whichcarries a cathode 9 which serves for the ECM of the metal component 1,which is arranged on a corresponding work holder 10. The cathode 9 is anozzle-like, narrow tube which can be moved in any desired manner inspace via the robot 8, such that any desired three-dimensionalstructures can therefore be traversed and machined. In addition to themobility of the cathode 9, it is also possible for the work holder 10 tobe movable, either translationally along one or more space axes orrotationally about one or more space axes, or both translationally androtationally, as indicated by the motion arrows.

A liquid electrolyte, for example an NaCl solution, is delivered via thecathode 9 directly into the working region, for which reason theelectrode 9 is embodied, as described, as a nozzle or tube. Provided forthis purpose is an electrolyte reservoir 11, from which the electrolyte12 is directed to the cathode 9 via a controlled pump 13 and a suitableelectrolyte feed line 14. Provided at the robot 8 is a correspondingconnection box 15, at which the line opens out and at which the cathode9 is also interchangeably accommodated. The volumetric flow of theelectrolyte can be monitored via a flow meter 16, and the fluid pressurecan be monitored via a pressure gage 17. Furthermore, a temperaturemeasuring device 18, a heating controller 19 and a pH measuringinstrument 20 and a conductivity measuring instrument 21 are provided inthe electrolyte reservoir 11 in order to be able to correspondingly setor monitor the electrolyte properties.

The electrolyte collected after delivery via the cathode 9 is fed backinto the electrolyte reservoir 11 by an electrolyte feed line 23, i.e. acircuit is established. The robot 8 and the work holder 10 are providedin an enclosure 24, i.e. the apparatus is closed to this extent withregard to the working region.

Furthermore, the apparatus comprises a process energy source 26, viawhich the working voltage and the process current can be applied. Theparameters are correspondingly monitored via an ammeter 27 and avoltmeter 28. A supply line 29 runs, once again, to the connection box15; it makes contact with the cathode 9. The supply return line 22 leadsfrom the metal component 1 back to the process energy source 26. In theprocess, the circuit is closed by the electrolyte stream.

Finally, a gas supply, in this case shown embodied as a compressed airsupply, for example in the form of a compressor 30, is provided, fromwhich an air feed line 31 runs likewise to the connection box 15. Thisair feed line is connected in turn to the cathode, which is embodied asa double-walled tube. The electrolyte is fed in the central passage; inthe outer passage, an air curtain which encloses the electrolyte can beblown out via the fed compressed air. A controlled restrictor valve 32and a flow meter 33, via which the air flow can be measured, areprovided in the air feed line 31.

Three roughly distinguishable regions are therefore provided, namely the“process energy” region A, the “electrolyte supply” region B and the“compressed air supply” region C.

Finally, a control device 34 is provided. The control device controlsthe operation of the robot 8, that is to say the free movement in spaceof the cathode 9 and the movement of the work holder 10, if provided. Itis of course also possible to control and monitor all the sub-componentsof the apparatus in FIG. 2 for the electrolyte and gas supply and theprocess energy source 26 (that is to say the regions A, B and C) via thecontrol device 34.

FIG. 3 shows, as a diagrammatic illustration, the tip of the cathode 9in a sectional view. The electrolyte 12 flows through the tubular,nozzle-like cathode 9. The anodically polarized metal component 1, forexample the guide vane cluster 2 known from FIG. 1, is at a distancefrom the cathode 9. As illustrated by the arrow diagram, the cathode 9can be moved translationally in the three directions in space, just asit can also be moved rotationally about each of the three directions inspace. The robot 8 is accordingly activated for this purpose via thecontrol device 34. Control is based here on a stored model of the metalcomponent 1, which defines the surface along which the electrode 9 is tobe moved.

It can be seen that the electrolyte 12 is conveyed through thenozzle-like or tubular cathode 9 and delivered to the metal component 1.An electric flow field 36 forms in the electrolyte stream 12.Electrochemical, locally limited metal removal takes place in the region37, i.e. a cavity forms in the metal component 1. The correspondingremoval depth is obtained in accordance with the process parametersselected.

FIG. 4 shows, as a diagrammatic illustration, the movement of thecathode 9, which, for example in the case of a round cross-sectionalgeometry, has a diameter of three millimeters and is at a distance of,for example, one millimeter from the original workpiece surface. It canbe seen that a linear region 37 can be removed by a horizontal movementof the cathode 9, as shown by the arrow P. Whereas only a movement alongone space coordinate is shown in FIG. 4, it is possible, as described,for the cathode 9 to be moved in any desired manner in space, i.e. theround edge regions of the guide vane cluster 2 shown in FIG. 1 or thethree-dimensionally twisted airfoils 5, etc., can be readily traversedin order to remove material there to the desired extent.

Finally, FIG. 5 shows a further embodiment of the cathode 9, which isembodied as a double-walled tube. The electrolyte 12 is directed in thecentral passage 38. The compressed air fed via the gas feed device,shown in FIG. 2 as air feed device 31, is discharged in the outerpassage 39. As FIG. 5 shows, a gas curtain 40 enclosing the electrolytestream 12 all around is formed. This reduces the effect on adjacentmetal component surfaces.

Even though a cathode 9 of round cross section is shown by way ofexample in the figures, the cathode can of course also have an elongatedcross section or any other desired cross section. It can have, forexample, in the electrolyte passage, a length of 10 mm and a width of 3mm, such that a long, but narrow, zone can be machined, which isexpedient in particular for machining relatively large areas. If a gascurtain is present, the corresponding air passage has, of course, acorresponding geometry.

1. A method for machining a metal component which has athree-dimensional shape produced by removing and/or shaping material,the method comprising the steps of: electrochemically machining at leastone superior component section using a nozzle-like cathode, via which anelectrolyte is delivered into a working region; and moving the cathodeor the metal component freely in space with a manipulator element. 2.The method according to claim 1, wherein the manipulator element is arobot having a plurality of motion axes.
 3. The method according toclaim 2, wherein the robot has five or six motion axes.
 4. The methodaccording to claim 1, including also moving the metal component arrangedon a movable work holder in the case of the cathode being moved by themanipulator element during machining.
 5. The method according to claim1, including also moving the cathode arranged on a movable holder in thecase of the metal component being moved by the manipulator elementduring machining.
 6. The method according to claim 1, includingdelivering the electrolyte as a stream of substantially round crosssection from a cathode having a corresponding cross-sectional geometry.7. The method according to claim 1, wherein a working voltage appliedbetween the cathode and the metal component or a process current flowingtherebetween is constant or pulsed and/or a volumetric flow of theelectrolyte is between 10 and 100 l/h.
 8. The method according to claim7, including determining a distance of the cathode from a surface of themetal component from a ratio of working voltage and process currentand/or, as a function thereof, and controlling a process sequence byvarying any desired process parameter.
 9. The method according to claim6, including blowing out a gas curtain laterally enclosing theelectrolyte stream at least partly via the cathode.
 10. The methodaccording to claim 1, wherein the metal component to be machinedconsists of a metallic or intermetallic material.
 11. The methodaccording to claim 10, wherein the metal component is a steel, atitanium alloy or a nickel alloy.
 12. The method according to claim 1,wherein the metal component is a component of a turbomachine.
 13. Themethod according to claim 12, wherein the component of a turbomachine isa blade component, in particular a guide vane cluster or a guide vanering.
 14. The method according to claim 13, wherein the superiorcomponent sections machined are edges and surfaces in a region betweentwo airfoils of the blade component and/or the airfoils of the bladecomponent themselves.
 15. An apparatus for implementing the methodaccording to claim 1, comprising: a manipulator element formed as amulti-axis robot; an ECM tool formed as a nozzle-like cathode or a workholder arranged on the manipulator; an electrolyte feed device forfeeding electrolyte from an electrolyte reservoir to the cathode; aprocess energy source connected to the cathode and the metal component;and a control device controlling operation of the apparatus.
 16. Theapparatus according to claim 15, wherein the cathode has a round crosssection.
 17. The apparatus according to claim 15, wherein the cathode isdesigned for delivering gas, fed to the cathode via a gas feed device,formed as a gas curtain laterally enclosing the electrolyte stream atleast partly, said electrolyte stream discharging from the cathode. 18.The apparatus according to claim 15, wherein the work holder isadditionally movable when the cathode is arranged on the manipulatorelement, or the cathode is additionally movable when the work holder isarranged on the manipulator element.
 19. The apparatus according toclaim 15, further comprising means for detecting a distance of thecathode from a surface of the component.
 20. The apparatus according toclaim 19, wherein the means is the control device, which determines thedistance with reference to a ratio of working voltage and processcurrent and, as a function thereof, controls a process sequence byvarying any desired process parameter.
 21. The apparatus according toclaim 19, wherein the means is a sensor, wherein the control devicecontrols a process sequence by varying any desired process parameter asa function of a detection result of the sensor.